The incidence of Q fever in the US is likely underestimated because infections are seldom diagnosed with confirmatory tests and clinical presentation is similar to a variety of fevers of unknown origin. Infections are not verified by culturing of the organisms because of difficulties inherent to propagation of the organism and the value of DNA identification of the organism during infection has not been established. Serodiagnostic testing currently relies upon reaction with whole organisms as antigen and assays are not well standardized and not readily adaptable to large-scale screening. The objectives of this application are to create recombinant serodiagnostic reagents and improved PCR detection methods to adequately prepare for identifying infected individuals. The working hypothesis of the serodiagnostic component is that unique specific protein antigens are recognized early in infection during acute Q fever. We have identified and cloned a variety of immunodominant proteins from C. burnetii. We propose to completely characterize the key antigens recognized during infection using proteomic and genomic approaches. The expressed recombinant antigens will be evaluated for their utility as serodiagnostic reagents initially in an ELISA format with recombinant His-tagged purified proteins. These reagents will then be compared with current assays for sensitivity and specificity using a large collection of human Q fever patient sera. The working hypothesis of the PCR detection component is that organisms can be detected in blood samples early in infection, prior to the development of clinical symptoms and a specific immune response. To test this hypothesis, we will establish an aerosol-delivered acute Q fever model, in guinea pigs, to accurately represent human disease caused by inhalation, the natural route of infection and the likely route of infection resulting from a bioterrorist release. Previous studies suggest that organisms can be routinely detected by PCR in blood samples from patients serologically confirmed to have acute Q fever but critical issues concerning when patients become bacteremic after infection and how long a bacteremic state exists have not been elucidated. PCR detection offers an attractive alternate approach to identify C. burnetii in blood since bacterial culturing is cumbersome, time-consuming, and requires BL-3 containment.